Semiconductor elements, and membrane sensors, in particular, as well as methods for producing membrane sensors on the base of semiconductor substrates such as silicon wafers are already known. For instance, flat porous membrane regions are arranged on the semiconductor substrate as substrate for sensor structures, and a cavity is produced underneath the membrane, e.g., for the thermal insulation of the membrane.
The membrane sensors currently on the market are mostly fashioned as thin film membrane sensors. For this purpose, layer systems in thicknesses of between several 10 nm and several μm are deposited on a carrier substrate, and the carrier substrate is subsequently removed in predefined areas in order to obtain self-supporting membrane areas. The structural elements of the sensor can then be arranged in the center of the membrane.
Surface micromechanics (SMM), in which a sacrificial layer is generally used that is deposited on the front side of a carrier substrate before membrane deposition, is another possibility for exposing the membrane. The sacrificial layer is later removed from the front side of the sensor through “detachment openings” in the membrane, whereby a self-supporting structure is created. These surface micromechanical methods are comparatively costly, on account of the necessity for separate sacrificial layers.
Published German patent document DE 100 32 579 discloses a method for manufacturing a semiconductor element and also a semiconductor element produced according to the method, in which, e.g., for a membrane sensor, a layer of semiconductor carrier material that was rendered porous is arranged above a cavity. Two layers having different porosities are formed to produce the cavity using appropriate etching parameters. Whereas the first layer has lower porosity and seals up during a subsequent first annealing step, the porosity of the second layer increases during the annealing step in such a way that a cavity is formed. In a second process step, at a higher annealing temperature, a relatively thick epitaxy layer as second membrane layer is grown on top of the first membrane layer formed from the first porous layer.
It may also be provided that a thin epitaxy layer be grown during the first annealing step in order to ensure complete sealing of the porous first layer, which is used as starting layer for the epitaxy growth of the thick epitaxy layer. In this context, a lower growth rate is selected at a lower temperature compared to the subsequent deposition of the thick epitaxy layer.
As a result of the mentioned measures, the construction of an SMM semiconductor element can be simplified considerably since no additionally deposited sacrificial layer is required and, furthermore, the membrane itself or an essential portion of the membrane is produced from semiconductor substrate material.
However, tests have shown that at least partially porous membrane may get damaged during manufacture, or that damage may not always be reliably prevented under normal operating conditions. To avoid damage to the membrane during manufacture or in regularly occurring application cases, published German patent document DE 101 38 759 provides a method for manufacturing a semiconductor element having a semiconductor substrate, in which the semiconductor substrate receives a different doping in the region of the porous membrane layer than the doping of the region of the subsequent cavity. After doping, the semiconductor material of the membrane layer is rendered porous, and the semiconductor material underneath the semiconductor material, rendered porous, is at least partially removed or relocated to provide a cavity.
Published German patent document DE 100 30 352 discloses a micromechanical component which has a support body made from silicon and a regionally self-supporting membrane which is joined to the support body. The membrane is regionally and superficially provided with at least one stabilizing element for support. To form the regionally self-supporting membrane, the silicon is rendered porous in a first region and is selectively removed via an etch opening once the membrane layer has been deposited.